Add a callback interface that allows interested parties to get notified whenever...

Add a callback interface that allows interested parties to get notified whenever PCHReader deserializes a type or decl (and possibly other things in the future). Have PCHWriter implement these callbacks as noops and register to receive them if we're chaining PCHs. This will allow PCHWriter to track the IDs of these things, which it needs to write the dependent files. WIP

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@108383 91177308-0d34-0410-b5e6-96231b3b80d8